The pattern shown on this afternoon’s GFS ensemble mean is interesting, showing a large, positively tilted trough moving through the western US. To the east of this trough, heights are rising over the Plain states, allowing warmer and more moist air to flow north. In addition, the North Atlantic Oscillation (NAO) is in a positive state, shown by well below normal heights over Greenland. This will allow any storms to cut well north over the Plains into southern Canada, which will open the door for warm and moist Gulf of Mexico air to stream well north ahead of the impressive piece of jet energy as it moves east.
The day 3 surface forecast valid Sunday morning from the Hydrometeorlogical Predication Center (HPC) shows a deepening low over the western Plains, with a long northwest to southeast oriented warm from extending all the way to the Gulf Coast. Also note the southeast ridge allowing warm and moist air to stream north from the Gulf into the Plains on the backside of the ridge.
This afternoon’s European Model (ECM) forecast valid the same time shows a potent piece of upper level jet energy rounding the base of the trough over the western US, getting ready to eject into the Plains by Sunday evening. In addition, the model appears to hint at some precipitation along the warm front (noted by the higher mid level relative humidities in the bottom panels) , however the model appears to be indicating the presence of an elevated mixed layer in place over the warm sector (southern Plains) Sunday morning, indicating that the environment will be fairly capped off until the aforementioned jet streak ejects east and helps break the cap.
By evening however, it appears that as the cold front, dry line, and nose of the upper level jet streak may begin moving east into the capped environment there could be enough lift to break the cap and allow storms, potentially supercells, to develop over the central Plains eastward towards the Mid Mississippi Valley.
An early look at the GFS model’s surface to 500mb wind crossover for Sunday evening reveals strong speed and directional wind shear over the warm sector Sunday evening, due to the strength of the surface low over the Plains. This, in combination with moderate to strong instability that will develop as a result of the elevated mixed layer, will result in super cells and tornadoes being likely where storms develop Sunday, if the cap breaks.
By Monday morning, the HPC shows the moderately deep surface low moving ENE through the Mid Mississippi Valley towards the lower Great Lakes, dragging a cold front through the southern Plains and extending a warm front east through the Ohio Valley towards the southern Mid Atlantic.
The ECM model forecast for Monday morning shows a very strong low level jet extending from the Gulf Coast north into the southern Great Lakes. This will allow the warm front to advance NE likely all the way into northern OH and PA by evening, and will assist in rapidly advecting Gulf moisture north all the way into the upper Ohio Valley. This may set the stage for a widespread severe weather outbreak for Monday, depending on other factors.
The other panels of the ECM model forecast still hint that some sort of an elevated mixed layer/cap may exist in parts of the warm sector, shown by the drier air extending north into the Ohio Valley Monday morning. This may cap things off enough for moderate to strong instability to again develop by Monday afternoon and result in significant severe weather when things get going.
The GFS model surface to 500mb wind crossovers for Monday evening again show a decent amount of turning in the warm sector, especially near the warm front and cold front.
In addition, note how the GFS has advected a decent amount of Gulf moisture well northward by Monday evening, with dew points well into the 60s as far north as the lower Great Lakes by Monday evening. A weak elevated mixed layer/cap may assist in forcing some solar heating of this moist air in the warm sector, resulting in sufficient instability for supercells and severe line segments, as shear will be strong in the warm sector.
By Tuesday, the HPC shows the surface low over extreme NE OH/NW PA with much of the Mid Atlantic, from NYC points south, in the warm sector.
The ECM model shows a very strong low level jet from the eastern Gulf coast north into the Mid Atlantic and even southern New England. This may set the stage for another round of severe weather, possibly significant, over the southeast and Mid Atlantic for Tuesday, with damaging winds, large hail, and tornadoes being possible due to the very strong wind shear that will be in place.
The Storm Predication Center is also worried about a significant severe weather outbreak, and has outlined large areas of possible severe weather for Sunday, Monday, and Tuesday (Days 4, 5, and 6 respectively), and these areas may need to be changed/expanded slightly north if the current models hold and the warm front gets to the lower Great Lakes and upper Mid Atlantic.
Flooding will also be an issue with this system as it moves east. A strong upper level jet out of the west-southwest over top of a strong low level jet out of the south-southwest combined with moderate instability just south of the warm front will cause an elevated risk of training convection near and north of the warm front, especially Sunday night-Monday night over parts of the Mid Mississippi Valley, upper Ohio Valley, and lower Great Lakes which could result in some flash flooding problems. Note how this afternoon’s GFS run painted a large area of heavy rainfall from Sunday-Tuesday:
More on this severe/flood threat over the coming days.